NGC 869 4024 , the SIMBAD biblio

We present uvbyβ CCD photometry of the central region of the double cluster h & χ Persei. We identify ≃350 stars, of which 214 were not included in Oosterhof's catalogue. Our magnitude limit V=16.5 allows us to reach early F spectral type and obtain very accurate fits to the ZAMS. We derive reddening values of E(b-y)=0.44±0.02 for h Persei and E(b-y)=0.39±0.05 for χ Persei. From the ZAMS fitting, we derive distance moduli V₀-M_V=11.66±0.20 and V₀-M_V=11.56±0.20 for h and χ Persei respectively. These values are perfectly compatible with both clusters being placed at the same distance and having identical reddenings. The shift in the main-sequence turnoff and isochrone fitting, however, show that there is a significant age difference between both clusters, with the bulk of stars in h Persei being older than χ Persei. There is, however, a significant population of stars in h Persei which are younger than χ Persei. All this argues for at least three different epochs of star formation, corresponding approximately to logt=7.0, 7.15 and 7.3.